1. Field of the invention
The present invention relates to a finite impulse response (FIR) filter for wave-shaping digital quadrature amplitude modulation (QPM) symbols, in which multipliers are replaced with multiplexers, the replaced multiplexers are utilized to receive the symbols directly from a symbol encoder without zero (0) interpolations, and the critical path is reduced by shifting the positions of delay devices.
2. Description of the prior art
Generally, in the digital 2.sup.k QAM method, successive k binary data are encoded into pairs of in-phase quadrature symbols, and are transmitted after wave-shaping, thereby reducing the transmission band into 1/k through the channel.
FIG. 1 is a circuit diagram showing the constitution of the general FIR filter.
The general FIR filter is a wave-shaping FIR filter of a 32-tab digital QAM transmitting section which satisfies the digital QAM 51,84 Mbps downstream transmission standard using symbol values of 1 and 3 of Class B which is recommended in DAVIC 1.2 spec. part 8.
The general FIR filter includes 31 modules (10-n) and a multiplier 104. Each of the modules 10-n includes a D flip flop, a multiplier and an adder. Therefore, the general FIR includes 32 multipliers, 31 adders and 31 D flip flops.
The multiplier 104 internally has a pre-set filter tab coefficient ho, and thus, the inputted symbol value is multiplied by the pre-set filter tab coefficient ho to supply the product to an adder 103 of a first module 10-1.
In the first module 10-1, a D flip flop 101 is used as a delay means to carry out a temporarily storing function. Further, it receives symbol values and clock signals through its input terminal to output one of the inputted symbols through an output terminal. A multiplier 102 multiplies one of the output symbols of the D flip flop 101 by the pre-set filter tab coefficient hl. An adder 103 sums up the output values of the multipliers 102 and 104 to supply the summed value to an adder 103-1 of a second module 10-2.
In the second module 10-2 having the same constitution as that of the first module 10-1, a D flip flop 101-1 serves as a delay device to receive clock signals and the output values of the D flip flop 101 of the first module 10-1 through its input terminal so as to output symbol values through its output terminal. A multiplier 102-1 multiplies one of the output symbol values of the D flip flop 10l-1 by a pre-set filter tab coefficient H.sub.2. An adder 103-1 sums up the output value of the multiplier 103-1 and the summed value of the adder 103 of the first module 10-1 to supply the summed value to an adder (not illustrated) of a third module (not illustrated).
The third to 31st modules have the same constitution as that of the second module 10-2, and carry out the same operations as that of the second module 10-2. In the 31st module 10-31, the final output values are obtained by multiplying the output symbol value of the D flip flop 101-30 by a filter tab coefficient h.sup.31 and by summing the summed value of the preceding adder (not illustrated) and the summed value of the adder 103-30.
Now the general FIR filter will be described as to its operations.
The input symbol (x[2:0]) is one of four numbers -3, -1, 1 and 3. One bit among the three bits is used as the code bit of the symbol, while the remaining two are used as the magnitude bits of the symbol.
In the case where the wave shaping filter consists of the general FIR, the FIR filter receives clock signals at a speed of 51.84 Mbps by interpolating three zeros (0) between the respective symbols to solve the problem of the mismatching between the output operating speed (12.96 Mbps) of a symbol encoder (not illustrated) and the input speed (51.84 Mbps) of the wave-shaping filter.
The critical path which represents the longest path from the delay device to the output stage for obtaining one signal requires one multiplier and 31 adders.
Therefore, the conventional general FIR filter has a complicated structure due to the multipliers and adders. Further, due to the difference between the output speed of the symbol encoder and the operating speed of the wave-shaping FIR filter, zeros (0) should be inserted into between the symbols so as to prevent the mixing of the data. Further, in order to obtain one output, a long critical path has to be gone through, with the result that a long time period is consumed in obtaining one output value.